How does water climb a 100-metre tree with no pump?

A tree has no heart, yet water reaches its highest leaves, as much as a hundred metres up. The trick is that the tree does not push water up from below. It pulls it from above, and the energy comes from evaporation.

Inside the trunk, water travels through the xylem, a network of tiny dead tubes. The tubes are not powered by the tree, because their cells are dead by maturity and have no living contents. The pull comes from the leaves. As water evaporates from the surface of leaf cells into the air, the water left behind recedes into minute pores in the cell walls and forms millions of tiny curved surfaces. The surface tension of water tugs on them and creates negative pressure, a suction, which pulls water from the roots and the soil.

That works because water molecules cling to one another. The slightly charged ends of neighbouring molecules form hydrogen bonds, the same attraction that gives water its surface tension. So when one molecule evaporates from a leaf, it tugs on the next, which tugs on the next, all the way down to the roots, like a chain pulled from one end. The idea is called the cohesion-tension theory and was proposed in 1894 by John Joly and Henry Horatio Dixon.

The result is that transpiration is a passive process that requires no energy expense by the plant. Evaporation does the work. Only a small amount of the water roots absorb is used for growth and metabolism: the remaining 97 to 99.5 percent evaporates, which is the price of keeping leaf pores open to take in CO2.